As prosecutors seek their first jury verdict in the Galleon Group insider-trading case, they've own their 20th guilty plea.

Jason Goldfarb, who was accused of conspiring with a former Galleon trader to buy confidential information from two other lawyers at his firm, Ropes & Gray, pleaded guilty on Thursday to securities fraud and conspiracy. He was set to face trial with four others in the middle of next month, alongside four others, including the former Galleon trader, Zvi Goffer.

According to prosecutors, Goldfarb was part of one of two interlocking insider-trading rings, in his case headed by Goffer. The other was allegedly headed by Galleon founder Raj Rajaratnam, who is on trial for his role in the case.

The other two Ropes & Gray lawyers, Arthur Cutillo and Brien Santarlas, pleaded guilty in January and December 2009, respectively. Cutillo said he received $32,500 from Goldfarb—money he believed came from Goffer—for providing confidential information about Ropes & Gray clients.

"I set up a dinner with Zvi Goffer, who explained to Mr. Cutillo that he was looking for any type of information he might come across that might benefit him and he might make some money," Goldfarb said in court. He said he "made a horrible mistake" by agreeing "to participate in this scheme."

Goldfarb faces between three and four years in prison when he is sentenced on Aug. 19. The trial of his former co-defendants is scheduled to begin on May 16.